Union Berlin is one of the most recognizable teams in the German capital, based in the Köpenick district. It was established in 1966, but its roots date back to the early 20th century. The club's home matches are played at An der Alten Forsterey, which has a capacity of 21,717 seats. The team's nickname is "Iron", and the anthem, which is sung every Christmas by more than 20,000 fans at the stadium, is called "Iron Union". Union's home kit is in red and white, while the reserve kit is usually in a reciprocal shade.

Like other German clubs, Union is the result of a merger of several teams. It is based on FC Olimpia Oberschöneweide, which was founded in 1906, but a few years later was renamed FC Union Oberschöneweide. The team was part of the elite of Germany, and its reputation as a working class team is impressive, unlike other metropolitan clubs such as Victoria and Blue Weiss 90 Berlin, which are associated with the aristocracy. After the partition of Germany, FC Union fell into the territory of East Berlin.

In the 1960s, the unification of Union Oberschöneweide with other teams in the capital seemed inevitable. This is a common practice in countries under the control of communist forces. Thus, the current 1. FC Union Berlin was created, but its beginning was not so successful. After the unification of West and East Berlin, the "Iron" remained outside the Second Bundesliga for a long time, playing mainly in the regional Oberliga. This changed only in 2001, when the Bulgarian striker Kostadin Vidolov was among the team's top scorers, and Georgi Vassilev was their manager.

Union Berlin's first played in the Bundesliga in the 2019/2020 season. The right to be part of the elite was won with a victory over Stuttgart in the decisive playoff. With this achievement, Union became the first club from East Berlin to play in the elite division of German football.

Union's biggest rival is BFC Dynamo, which is part of the GDR. The rivalry with Hertha Berlin is not strong, but will be watched with interest in the Bundesliga.